Hemoglobin is a protein in your red blood cells that carries oxygen to your body's organs and tissues and transports carbon dioxide from your organs and tissues back to your lungs.

If a hemoglobin test reveals that your hemoglobin level is lower than normal, it means you have a low red blood cell count (anemia). Anemia can have many different causes, including vitamin deficiencies, bleeding and chronic diseases.

If a hemoglobin test shows a higher than normal level, there are several potential causes — the blood disorder polycythemia vera, living at a high altitude, smoking, dehydration, burns and excessive vomiting.

You may have a hemoglobin test for several reasons:

To check your overall health. Your doctor may test your hemoglobin as part of a complete blood count during a routine medical examination to monitor your general health and to screen for a variety of disorders, such as anemia.

To diagnose a medical condition. Your doctor may suggest a hemoglobin test if you're experiencing weakness, fatigue, shortness of breath or dizziness. These signs and symptoms may point to anemia or polycythemia vera. A hemoglobin test may help diagnose these or other medical conditions.

To monitor a medical condition. If you've been diagnosed with anemia or polycythemia vera, your doctor may use a hemoglobin test to monitor your condition and guide treatment.

If your blood sample is being tested only for hemoglobin, you can eat and drink normally before the test. If your blood sample will be used for other tests, you may need to fast for a certain amount of time before the sample is taken. Your doctor will give you specific instructions.

For a hemoglobin test, a member of your health care team takes a sample of blood by pricking your fingertip or inserting a needle into a vein in your arm. For infants, the sample may be obtained by pricking the heel. The blood sample is sent to a lab for analysis. You can return to your usual activities immediately after the sample is taken.

The normal range for hemoglobin is:

For men, 13.5 to 17.5 grams per deciliter

For women, 12.0 to 15.5 grams per deciliter

Normal ranges for children vary with age and sex. The range for a normal hemoglobin level may differ from one medical practice to another.

Lower than normal results

If your hemoglobin level is lower than normal, you have anemia. There are many forms of anemia, each with different causes, which can include:

If you've been previously diagnosed with polycythemia vera, an elevated hemoglobin level may indicate a need to alter your treatment plan.

If your hemoglobin level is below or above normal, your doctor may want to evaluate the hemoglobin test results along with those of other tests, or additional tests may be necessary, to determine next steps.

For specifics about what your hemoglobin test results mean, talk to your doctor.

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